Field of the Invention
The present invention relates to a finder system and an optical device having the same and, more particularly, to a finder system suitable for an optical device such as a still camera, a video camera, or a digital camera which observes, with an eyepiece lens system, a finder image (object image) of a real image formed on a predetermined surface by an objective lens system at a variable magnification.
Related Background Art
In recent years, a real-image finder system capable of clearly observing the field frame (finder field frame) is widely used as the finder system of a camera such as a still camera or video camera in place of a virtual-image finder system of reverse-Galilean scheme.
Lens configurations of real-image finder systems are roughly classified into a primary imaging type system comprising an objective lens system, an erecting optical system for inverting the reverse image (inverted image) of an object, which is formed by the objective lens system, into an erected image using a prism such as a porro-prism or roof prism, and an eyepiece lens system disposed on the rear side of the erecting optical system to simultaneously observe the erected image and the field frame for limiting the observation range, and a secondary imaging type system using, in place of a prism as an erecting optical system, a secondary imaging system for re-imaging the reverse image of an object, which is formed by an objective lens system, to convert the reverse image into an erected image.
The finder magnification of a real-image finder system is generally defined as follows.
In a primary imaging type real-image finder system, letting fo be the focal length of the objective lens system, and fe be the focal length of the eyepiece lens system, a finder magnification xcex31 is defined by
xcex31=fo/fexe2x80x83xe2x80x83(1)
In a secondary imaging type real-image finder system, letting fo be the focal length of the objective lens system, xcex2 be the magnification of the secondary imaging system, and fe be the focal length of the eyepiece lens system, the finder magnification xcex32 is defined by
xcex32=xcex2xc2x7fo/fexe2x80x83xe2x80x83(2)
That is, the magnification of the secondary imaging type finder system is obtained by multiplying the magnification of the primary imaging type finder system by the magnification xcex2 of the secondary imaging system. Since the secondary imaging magnification xcex2 can be arbitrarily set, the secondary imaging type finder system can increase the degree of freedom in designing the finder magnification.
The type of real-image finder system is generally selected in accordance with the form of a camera. To decrease the thickness of a camera, the primary imaging type camera is used. To increase the thickness of a camera, the secondary imaging type is used.
In the primary imaging type, the extending direction of the finder optical system changes depending on the prism used, and various types of finders can be selected in accordance with the camera form and use form.
However, the positional relationship between the observation optical axis and the position of optical axis of incident light on the finder is determined by the type of prism used and cannot be freely changed.
Additionally, in terms of camera specifications, when the secondary imaging type finder having a high degree of freedom in designing the finder magnification is to be applied to a thin camera, the optical path of the finder must be appropriately bent using a planar mirror and the like, and the number of components tends to inevitably increase.
Recently, as image sensing lenses capable of zooming become popular, finder systems are also being equipped with a zooming function.
For zooming in a real-image finder system,
(A) the magnification is changed in the objective lens system,
(B) the magnification is changed in the eyepiece lens system,
(C) the magnification is changed in the secondary imaging system, or
(D) the magnification is changed by combining (A) to (C).
A real-image finder system has a high observation quality because it allows the user to more clearly see the field frame as compared to an virtual-image finder system, though the number of lens components tends to increase in principle.
The number of lens components increases especially when the finder system should have a zooming function.
To decrease the number of lenses in the conventional real-image finder system,
an aspherical lens is used, or
a prism and lens are combined.
However, the number of lenses can be only limitedly decreased using an aspherical lens. In addition, the composition of a prism and lens can only add a field lens function to the incident or exit surface of the prism.
Furthermore, in the finder system of a camera, the material of a lens in the finder system is often made of only acrylic or polycarbonate resin because of cost, moldability, and optical characteristics, and there is no degree of freedom in selecting the material. For this reason, various aberrations and particularly, chromatic aberration can hardly be corrected in a finder system having a zooming function.
It is an object of the present invention to provide a finder system suitable for a still camera or video camera, which allows a user to satisfactorily observe a finder image (object image) at various finder magnifications while simplifying the entire optical system using an appropriately set optical element in a finder system for allowing the user to observe an object image, and an optical device having the finder system.
In order to achieve the above object, according to the present invention, there is provided an observation optical system comprising an objective optical system for forming an inverted image of an object, an erecting optical system for converting the inverted image into an erected image, and an eyepiece optical system for guiding the erected image to an observer, wherein at least one of the objective optical system, the erecting optical system, and the eyepiece optical system has a first optical element having a curved reflecting surface and changes an observation magnification by changing the relative positions of the first optical element and another optical unit.
According to the present invention, there is also provided an optical device comprising the observation optical system of the present invention.